1. Field of the Invention
Aspects of the invention are related to input circuits that reduce dispersion of input resistances to settle to a specified value.
2. Description of the Related Art
FIG. 7 shows a connection construction example of a conventional igniter. Referring to FIG. 7, a signal is given from an electric control unit (ECU) 1 through a connector 2 to an igniter 3. An insulated gate bipolar transistor (IGBT) 7 installed in the igniter 3 conducts ON/OFF operation of the primary current through a coil 4 to generate a voltage at several tens of kilovolts across the secondary winding of the coil 4. This voltage generates a discharge between both sides of a gap 5 of a spark plug to ignite engine fuel. The ECU 1 in FIG. 7 can detect the current Iin through the signal line in order to avoid ignition failure due to poor contact at the connector 2.
Japanese Unexamined Patent Application Publication No. 201 1-1 20474 (also referred to herein as “Patent Document 1”) discloses a circuit for supplying power to an intrinsically safe load. The circuit comprises a power source, output terminals connecting to the intrinsically safe load, a voltage limiting circuit between the power source and the output terminals, and a current limiting circuit that converts a current to a voltage for comparison in an operational amplifier and includes a barrier resistor for limiting the current to the load. The voltage limiting circuit limits the voltage across the load, and the current limiting circuit includes an instantaneous current limiting circuit to limit an instantaneous current to the load. The voltage limiting circuit and the current limiting circuit supply electric power to the load.
The ECU 1 shown in FIG. 7 needs to detect a slight variation in current for detecting precursory phenomena of poor contact at the connector 2. Thus, the input resistor 6 of the igniter 3 needs to have an accurate value. Because an accuracy that is attained through a usual semiconductor process is insufficient for the input resistor 6, the accuracy of the input resistor is generally achieved by composing the input resistor using a discrete component and by mounting the input resistor on a ceramic substrate (not shown in the figure).
Thus, in conventional igniters, the power device such as an IGBT is mounted on a copper frame (not shown) exhibiting good heat dissipation, and a discrete component and integrated circuit (IC) for control are mounted on a ceramic substrate, which is expensive.
To detect a feeble contact resistance, the input resistor 6 is set at a relatively small resistance value. As a consequence, if input terminals are short-circuited with the power source such as a battery due to misconnection at the connector 2 or inter-connection caused by foreign matter in contact with the terminals, a heavy current flows that may cause inflammation which is a serious accident for a vehicle.
Patent Document 1 does not describe a function virtually equivalent to a current limiting resistor performed by controlling a constant current source with an input voltage.